Ian,
Thanks for the pointers! I could delete the relays since I am going to
dedicate the IC-251A to low power service on a permanent basis. I
bought the rig specifically to be used with transverters. I have a much
newer rig that covers 2M (Yaesu FT-857) and it puts out 50 Watts instead
of only 10 Watts like the IC-251A.

Now it's just a matter of where to "pick off" that low level RF. I have
it narrowed down to Q30 (pre-driver), but there is so much circuitry
(coils and caps) between it's collector and the next stage's base lead.
Do you think I could just remove the driver stage (Q31) and solder the
center lead of a piece of coax, say RG-174, into the now empty base lead
hole and bring it out the back of the rig? Is any sort of impedance
matching required?

The plan is to use this to drive the Down East Microwave series of
transverters, so harmonics may not be a real big issue since I believe
they have pretty tight filtering at their design frequencies that would
greatly attenuate the next "spur" 144 MHz above the desired output (i.e.
1440 MHz for the 1296 transverter)...

Any final thoughts???

Scott
N0EDV




Ian White, G3SEK wrote:
Allison wrote:


First pull the jumper.
Second pick up the rf at the last "live" stage and bring it to a jack.
I assume the transverter will permit seperate TX and RX paths, the
better ones do. If the pickoff point is more than you need still the
attenuator will not have to be big or high power.

As to just pulling the plug and hoping fo rf leak through, try it it
may work, my bet is no.

The band pass filter for the operating freqency is before the driver
and final so it's clean there. The transverter will also clean up the
input usually. So the level of excess harmonics should not be a problem.

Allison
KB1gmX


Allison has touched all the important bases - his suggestion is the best
way to do it.

The idea of just pulling the plug will not work - you'll turn all the
power devices into passive diode limiters, and if anything comes out at
all, it will be filthy. Likewise the huge reduction in audio drive will
not work, because the SSB/CW signal will drop down into the circuit
noise, and probably well below the level of the not-quite-suppressed
carrier.

For a complete solution, install a little RF relay to switch the drive
signal to an output socket on the rear of the transceiver. As Allison
says, do this *after* the filters at the operating frequency, just
before the signal enters the driver/PA boards. Also install a DC power
relay to remove the supply to those RF power stages. Wire the two relays
so that when they are not energized, the transceiver works as normal.

The deluxe - and safe - way to control those relays is to supply 12V DC
down the coax cable between the new output socket and the transverter
(using RF chokes and blocking capacitors to separate the DC from the RF
at each end). When the transverter is powered-on, it commands the
transceiver to send it low power. When the transverter is off, the
transceiver simply goes back to normal.

You don't know exactly what power level the driver stages will provide.
If it's too high, attenuate it. If it's too low, use an MMIC amp. If -
as seems very likely - the transverter comes from Down East Microwave,
the facilities you need will probably be on-board.

As Allison says, use a separate route for RX input. This will require
yet another small RF relay (powered the same way as the others) but it
will make sure you can never empty the full transceiver power into the
RX output of the transverter.

This system with DC control down the low-level TX coax will take a
little time to install, but it is highly reliable and very safe. I've
used it on a number of rigs here over the past 20+ years, and it has
never let me down.